Hierarchical hollow silicalite encapsulated Cu-Fe oxides for selectively oxidative depolymerization of lignin to diethyl meleate

Abstract

Heterogeneously catalytic depolymerization of lignin into value-added biochemicals are imperative, yet challengeable by the limited mass transport of lignin and the non-ideal spatial distributions of active sites in catalysts. Herein, a series of polycrystalline materials with Cu and Fe oxides encapsulated in hierarchical hollow nano silicalite (Cu_x-Fe_y@HhNS) were designed for step-by-step oxidative conversion of lignin into diethyl meleate (DEM). 92.0 % conversion of lignin with an exceptional DEM yield of 31.2 wt% and selectivity of 70.7 % was achieved at 150 °C for 24 h using Cu_2.5-Fe_2.2@HhNS. A series of controlled experiments and characterization showed clearly that the superior performance in DEM production was attributed to the enhanced tandem processes of lignin C_α-C_β bonds deconstruction by framework Cu²⁺, subsequently aromatic ring-cleavage by the isolated framework Fe³⁺, and the well-balanced micro-/mesoporosity ratio of Cu_x-Fe_y@HhNS facilitates the diffusion of lignin macromolecules/products, contributing to the excellent DEM yield and selectivity as well. Further mechanistic investigation illustrated that the process proceeds via a single electron transfer pathway. Consequently, this work provides new insights into lignin valorization to bulk chemicals, paving the way for biomass-derived recyclable polymeric materials as sustainable alternative to traditional petroleum-based routes.